The known meteorite samples originated mainly from the main asteroid belt where the average densities of objects are much higher than in the outer Solar System (\(\rho \)  \(\ge \)  2 g cm\(^{-3}\)  in contrast with the transneptunian region where even densities of \(\rho \)  \(\le \)  1 g cm\(^{-3}\)  are common).
In large Kuiper belt objects current estimates point to a common primordial (bulk) density of \(\sim \) 1.8 g cm\(^{-3}\)   [1]}, [2]}, indicating a rock-to-ice ratio of \(\sim \) 42:58 in volume, and \(\sim \) 70:30 in mass. This is notably higher ice content than in the main belt, where e.g. the \(\rho \)  = 2.16 g cm\(^{-3}\)   density of (1) Ceres [3]} indicates of rock-to-ice mass ratio of \(\sim \) 80:20, and the high \(\rho \)  = 3.5 g cm\(^{-3}\)   density of (4) Vesta [4]} suggests a very low water ice content. If the primordial density in the Kuiper belt was really close to the value obtained from the bulk density of large objects (\(\sim \) 1.8 g cm\(^{-3}\) ), small bodies in the D \(<\)  500 km range should have a notable macroporosity to have the observed bulk densities below \(\sim \) 1 g cm\(^{-3}\)  [2]}. The abundance of ice and the level of porosity indicate that aqueous alteration processes may have played an important role in the evolution of these objects, at least for a short time, early in the evolution of the Kuiper belt, when radiogenic decay provided enough heat for these reactions to start.
